The present invention relates to an exposure control system which controls the video signal of an object image by controlling the exposure time of a solid state image sensing device and the gain to be given to the output of the solid state image sensing device.
A conventional exposure control system controls the exposure time and the gain by providing an output from a solid state image sensing device with a gain, and, based on the value of this output, driving an iris diaphragm, a liquid crystal filter or such other means.
An example of a conventional exposure control system is described in Japanese Laid Open Utility Model No. Hei 1-159470.
A conventional exposure control system as disclosed by the above is described as follows:
FIG. 14 is a block diagram of a prior art exposure control system. In FIG. 14, numeral 1401 denotes a lens for receiving and focusing an object image, 1402 is a transparency variable liquid crystal filter, 1403 is a solid state image sensing device, 1404 is a control circuit for controlling the transparency variable liquid crystal filter, 1405 is a video amplifier, 1406 is an electronic shutter control circuit, and 1407 is a video amplifier with AGC.
The operation of an exposure control system constituted as above is described as follows:
An object image formed by lens 1401 through transparency variable liquid crystal filter 1402 is converted into an electric signal by solid state image sensing device 1403, which signal is taken out as a video signal via video amplifier 1405. Based on the level of the video signal, control circuit for controlling transparency variable liquid crystal filter 1404 adjusts the transparency of the transparency variable liquid crystal filter, electronic shutter control circuit 1406 adjusts the exposure time of solid state image sensing device 1403, and video amplifier with AGC 1407 adjusts the value of the gain resulting in the control of the exposure time and gain of the camera.
When the object brightness increases, the level of the video output signal of solid state image sensing device 1403 rises, and as a result, the level of the video output signal of video amplifier 1405 increases. In response, the light transmission of transparency variable liquid crystal filter 1402 is controlled to reduce the light transmission. When the object brightness increases further to exceed the control range of transparency variable liquid crystal filter, electronic shutter control circuit 1406 is put into operation to decrease the time for accumulating an electric charge in solid state image sensing device 1403, in order to lower the level of the video output signal.
In the above described conventional constitution, however, the transparency variable liquid crystal filter 1402 requires a certain size, making it difficult to make the whole system smaller. Moreover, because the electronic shutter works within a certain time after the final pulse of the discharge pulse is input within a horizontal blanking period for discharging the electric charge accumulated in the light sensing elements(photo diode) of the solid state image sensing device until a transfer pulse for transferring the electric charge accumulated in the light sensing elements to transfer channel as the exposure time of solid state image sensing device, the exposure time is a discrete value depending on each respective horizontal scanning period.
The discharging pulse is not input during the picture scanning period because during that period the signal is taken out through the transfer channel; if a discharge pulse is applied to the light sensing elements on the same substrate the signal is affected by the pulse and noise is caused on the picture. Therefore, the discharging pulse is limited to the horizontal blanking period to avoid the noise. As the exposure time of the solid state image sensing device decreases, the ratio of the discharging pulse period versus exposure time becomes larger. As a result the quantity of exposure shifts greatly with the input of a discharging pulse. Therefore, the output of the solid state image sensing device shows a step change when the exposure time is varied Consequently, the brightness of a picture flickers and presents a very unpleasant scene.